1. Field of the Invention
The present invention relates to a manufacturing method of a semiconductor memory device, and more particularly relates to a manufacturing method of a semiconductor memory device that uses a ring-gate transistor.
2. Description of Related Art
In a DRAM (Dynamic Random Access Memory), a sense amplifier is connected to each bit line. Although a sense amplifier is a circuit used to amplify a potential difference between a pair of bit lines, the sense amplifier is required to have high sensitivity because the potential difference between the bit lines is very small. Therefore, high current driving capacity is necessary, and current driving capacity needs to be secured by increasing to some extent a channel width of each of transistors constituting a sense amplifier. However, an occupied area of the sense amplifier is limited because of recent downscaling of DRAM cells. Accordingly, a technique of constituting a sense amplifier by using a ring-gate transistor, which is capable of taking a large channel width in a relatively small occupied area, is used in recent years (for example, see Japanese Patent Application Laid-open Nos. 2003-068880, 2005-347578, and 2008-016749).
When a ring-gate transistor is used, a contact plug that connects source/drains region to bit lines is formed by using a technique called a SAC (Self Aligned Contact) hole technique. According to this technique, an upper surface and a side surface of a gate electrode are first covered with a gate gap and a sidewall insulating film made of a silicon nitride. Next, the entire sense amplifier is covered on this with a silicon oxide film. The silicon oxide film is selectively etched by using a mask, thereby providing holes on the source/drain regions. A conductive layer is then embedded into the contact holes, thereby forming the contact plugs in self alignment manner.
The SAC hole technique has this name because a mask used for the above selective etching has holes for respective contacts.
However, the above technique has a problem in that a distance between a contact plug and a gate electrode becomes long. That is, because contact holes are very narrow holes, it takes some time before sufficient holes are formed. This is because it takes time for etchant to move in narrow holes. Accordingly, to prevent gate electrodes from being damaged, a gate gap and a sidewall need to have a large thickness to some extent. Consequently, a distance between a contact plug and a gate electrode becomes long corresponding to the large thickness.
When a distance between the contact plug and the gate electrode is increased, the entire size of a transistor is enlarged. This is a cause of hindering downscaling of a DRAM in its entirety, that is, reduction of a chip size.
Such a problem can occur not only to a sense amplifier of a DRAM but also to other types of circuits that use a ring-gate transistor.